Osmia lignaria, commonly known as the orchard mason bee or blue orchard bee,[1] is a megachilidbee that makes nests in reeds and natural holes, creating individual cells for their brood that are separated by mud dividers. Unlike carpenter bees, they cannot drill holes in wood. O. lignaria is a common species used for early spring fruit bloom in Canada and the United States, though a number of other Osmia species are cultured for use in pollination.

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O. lignaria is among 4000 native bee species of North America, and its species is divided by the Rocky Mountains into two subspecies, O. l. propinqua and O. l. lignaria. For those who keep mason bees, USDA entomologists have strongly suggested O. l. lignaria is kept because of its native origin, as the bees are likely to have greater success in their original climate and in propagation[disambiguation needed] efforts.

The bees begin to emerge from their cocoons in the spring when the daytime temperature reaches 14 °C (57 °F).[2] The males emerge first. They remain near the nesting site and wait for the females to begin their emergence, which can be several days to weeks depending on the number of days of warm weather. The first thing the females do is mate. A female will typically mate once, maybe twice. She will be absent from the nesting site for several days while she feeds and waits for her ovaries to fully mature.

"Bee house" used for Osmia lignaria

Orchard mason bee on an apple bloom

Example of nesting site variations

When a female is ready, she seeks out a suitable nest. O. lignaria females nest in narrow holes or tubes, though they have been found to nest inside cedar shakes and even keyholes. Beekeepers place prepared nesting materials to entice the females to stay close to the orchard or nearby forage. Good nesting material (reeds, paper tubes, wood trays or "bee condos") are as important as having the proper mud available (silty/clayey, as well as correct moisture content to grab/pack the mud). A female might inspect several potential nests before settling in. Once she has found a preferred nesting cavity, she flies outside of the hole and does an in-flight dance. She is orienting on major visual features to find her nest when she returns from foraging.

Orchard mason bees arrange their nests as a series of partitions, with one egg per partition. A female begins the process by collecting mud and building the back wall, if necessary, of the first partition. She then makes several back-and-forth trips to nearby flowers. Unlike honey bees, which visit flowers that are miles away, females visit flowers that are nearest the nest. One bee can visit 75 flowers per trip, and it takes 25 trips to create a complete pollen/nectar provision. The female works tirelessly during the day, only stopping once the sun has gone down. When the sun rises the next morning, she basks in its rays until warm enough to fly, then continues foraging.

Once the pollen provision is large enough, she backs into the hole and lays an egg directly upon it. She then collects more mud to seal off the partition. The new wall also doubles as the back wall of the next cell, and she continues until she has filled the nest hole with a series of offspring. O. lignara bees, like many insects, can select the gender of the egg they lay by fertilizing the egg, or not. Unfertilized eggs are males, while fertilized eggs are females. The adult bee lays female eggs in the back of the burrow, and the male eggs towards the front. On average, she lays about three males and one to two females per cavity. Because females are larger than males and require more pollen reserves, cavity dimensions can play a significant role in the cavity selection process.

When the egg hatches, the larva consumes the food provision and goes through many changes before becoming an adult. It will spend most of its life alone in this dark cell made by its mother.

Once the female has finished the nest, she plugs the entrance with a mud wall, thicker than the partitions that precede it. She then seeks another location for a new nest. She works tirelessly until she dies. An O. lignaria female lives for about four to eight weeks, and can fill an average of four six-inch tubes in her lifetime, with about eight eggs per tube. Her work includes nearly 60,000 blossom visits, and has attracted growers to propagate the insect for pollination purposes in fruit orchards.

The young bee is now a fully developed insect and undergoes diapause inside its cocoon for the duration of the winter. To stay warm, they burn through their fat reserves. If the weather stays cold for too long, the bees can die of starvation. Alternatively, if the temperature rises too fast, emergence may occur prematurely when pollen is scarce or the weather can return to cold temperatures for too long. Farmers are known to exploit the emergence cycle and time their release to coincide with the first orchard blossoms.

Orchard mason bees, like all mason bees, are very shy and will only sting if they perceive serious danger. They will not attack to defend themselves. The stinger is actually an egg guide. Because of their docile behavior, mason bees are preferred by people who desire pollination in urban settings.